EP-A-141927 (Beecham Group p.l.c.) discloses penciclovir, the compound of formula (A): ##STR1## and salts, phosphate esters and acyl derivatives thereof, as antiviral agents. The sodium salt hydrate of penciclovir is disclosed in EP-A-216459 (Beecham Group p.l.c.). Penciclovir and its antiviral activity is also disclosed in Abstract P.V11-5 p.193 of `Abstracts of 14th Int. Congress of Microbiology`, Manchester, England Sep. 7-13 1986 (Boyd et. al.).
Orally active bioprecursors of the compound of formula (A) are of formula (B): ##STR2## and salts and derivatives thereof as defined under formula (A); wherein X is C.sub.1-6 alkoxy, NH.sub.2 or hydrogen. The compounds of formula (B) wherein X is C.sub.1-6 alkoxy or NH.sub.2 are disclosed in EP-A-141927 and the compounds of formula (B) wherein X is hydrogen, disclosed in EP-A-182024 (Beecham Group p.l.c.) are preferred prodrugs. A particularly preferred example of a compound of formula (B) is that wherein X is hydrogen and wherein the two OH groups are in the form of the acetyl derivative, described in Example 2 of EP-A-182024, hereinafter referred to as famciclovir.
The compounds of formula (A) and (B) and salts and derivatives thereof have been described as useful in the treatment of infections caused by herpes viruses, such as herpes simplex type 1, herpes simplex type 2, varicella-zoster and Epstein-Barr viruses.
Zoster associated pain (ZAP) is considered to consist of the pain associated with zoster infection and includes the acute phase pain and post-herpetic neuralgia (PHN), which is by far the most common complication of herpes zoster infection and one of the most intractable pain disorders (Strommen et al, Pharmacotherapy, 1988; 8:52-68). Patients who develop PHN suffer from a debilitating and often intractable pain which can persist for months or even years. Although rare in patients under 50 years of age, the frequency of PHN rises steeply with increasing age.
There is currently no proven therapy for preventing PHN. The pain is due to injury of the nervous system and therefore seldom responds to analgesia used to treat pain associated with tissue damage. Hence, there is a need for therapy which alleviates or shortens the duration of post-herpetic neuralgia.